Bowling balls employ a variety of drill patterns for three-hole layouts. Two upper finger holes are separated laterally from each other by a bridge distance. The thumb hole is separated vertically from the finger holes by a span distance. Depending on a bowler's preference, the finger hole pattern may be drilled at an off-center position relative to the center of gravity of the ball to achieve a desired influence on ball trajectory. This allows bowlers to have a preferable amount of tracking, or curved trajectory, on the ball's approach toward bowling pins.
Bowlers may have a range of release types that also influence ball trajectory. A bowler with high speed and little hand rotation will have relatively low hooking action, particularly toward the back end of the roll. Likewise, a bowler with a lower ball speed and more hand rotation will tend to have much larger hooking action and a stronger back-end hook. During the initial portion of a ball approach, the force related to ball linear velocity may greatly outweigh the rotational force, and the ball may skid in a relatively straight direction while rotating in an oblique direction. During a middle portion of the ball approach, the force from oblique rotation influences ball trajectory, causing a hooking pattern of motion. Once the pattern changes, the ball begins to roll more in an oblique direction to approach the pins from an indirect angle.
The release type of a bowler's throw may make it desirable for a custom finger interface for the bowler to have more consistent control over the release. Unique finger hole shapes may be suitable to enhance bowler comfort as well as ball control. A custom finger interface may be beneficial for the finger and/or the thumb holes of a bowling ball.